The proposed research is concerned with investigations on the action of inhibitors of DNA biosynthesis as cancer chemotherapeutic agents. The compounds to be studied initially will be methotrexate, arabinosylcytosine, 1-formylisoquinoline thiosemicarbazone and isopentyladenosine as representatives of this class of agents. The objectives of this work are threefold: 1) to clarify where necessary the molecular basis for the action of each drug in particular tissues; 2) to provide a detailed explanation for the pharmacodynamic behavior of each drug in animals and in man, in order to predict the extent and duration of the period of effective action in particular tissues; 3) to use this information to explain the therapeutic efficacy of each drug in favorable situations and to attempt to improve its effect in others. Much of the work on metabolic mechanisms will be performed in cell culture, using contrasting lines of both human and mouse cells. Particular methods to be used involve chromatographic separations of folate cofactors, deoxyribonucleotides and ribonucleotides. Correlations will be established in animals between dosage regimen as reflected by the time course of blood concentration of a drug and the functional impairment of metabolism which results in both normal and tumor tissues. Attempts will be made to measure this functional impairment by methods which can be eventually applied to human patients.